Polymer PTC element

ABSTRACT

A polymer PTC element includes:  
     an element body including a polymer and a conductive material dispersively mixed in this polymer; and a pair of electrode foils in each of which at least one surface is roughened and this surface is applied with an Au flash plating and which are bonded on both surfaces of the element body respectively with the roughened surfaces facing the element body.  
     Therefore, electrical resistance in connecting portions of the element body and the electrodes is sufficiently lowered and the electrical resistance is not increased due to environmental and aging changes and so on.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a polymer PTC element whichshows a positive resistance temperature coefficient, for protecting abattery and a circuit against overcurrent, and more particularly, to apolymer PTC element appropriate as an element for protecting a circuitconnected to a battery pack of a cellular phone, a video camera, acomputer, and so on against overcurrent and overheat.

[0003] 2. Description of Related Art

[0004] A polymer PTC element is considered to be a kind of a PTC(Positive Temperature Coefficient) element. As a conventional example ofthis polymer PTC element, such a polymer PTC element is known which isproduced in a method disclosed in Japanese Patent Laid-open No. Hei8-246183, and in which electrolytic copper foils or electrolytic nickelfoils, each having one roughened surface, are bonded on both surfaces ofan element body respectively with the roughened surfaces facing theelement body to form electrodes of the element, or electrolytic copperfoils subjected to surface-roughening and electrolytic nickelictreatment are bonded on both surfaces of the element body respectivelywith the roughened surfaces facing the element body to form electrodesof the element.

[0005] Furthermore, as another conventional example of this polymer PTCelement, the one disclosed in Japanese Utility Model Laid-open No. Hei2-146401 is known. In this official gazette, disclosed is the structurein which soft-solder plating layers 118 are melted to connect electrodes114 provided in an element body 112 and lead terminals 116 with eachother by soft soldering as shown in FIG. 7.

[0006] However, the electrodes formed by the method disclosed inJapanese Patent Laid-open No. Hei 8-246183 has a disadvantage that it iseasily degraded since it is formed of copper or nickel and is subject tooxidation and so on so that electrical resistance in connecting portionsbetween the electrodes and the element body is increased in accordancewith the elapse of time. Especially, when copper is used to form theelectrodes, the increase in the electrical resistance is prominent,though it costs low.

[0007] Meanwhile, especially when lead terminals and electrodes aresimply connected by ordinary soft soldering in a polymer PTC elementoperating at a low temperature, there exists a disadvantage that a poorcharacteristic is caused due to thermal degradation of the polymer PTCelement since melting temperature of a soft solder is about 200° C. orhigher. Moreover, even when the soft-solder plating layers 118 aremelted to connect them as in Japanese Utility Model Laid-open No. Hei2-146401 as described above, the problem of the thermal degradationcannot be basically solved since melting temperature is also high inthis case.

SUMMARY OF THE INVENTION

[0008] In consideration of the above-described facts, it is a firstobject of the present invention to provide a polymer PTC element whoseelectrical resistance in a connecting portion between an element bodyand an electrode is sufficiently low, and in addition, which has astable electrical characteristic causing no increase in the electricalresistance in this connecting portion even with environmental changes,aging, and so on. Furthermore, it is a second object of the presentinvention to provide a polymer PTC element not only eliminating thedisadvantage of causing a poor characteristic due to thermal degradationbut also satisfying solderability of a lead terminal.

[0009] According to one of the aspects of the present invention,provided is a polymer PTC element comprising: an element body includinga polymer and a conductive material dispersively mixed in this polymer;and a pair of electrode foils in each of which at least one surface isroughened and this surface is applied with an Au (gold) flash platingand which are bonded, with these surfaces being faced with the elementbody, on both surfaces of the element body respectively.

[0010] The polymer PTC element as described above brings about thefollowing effect.

[0011] The polymer PTC element according to this aspect is so structuredthat it comprises the element body including the polymer, and theconductive material dispersively mixed in this polymer and the pair ofthe electrode foils are bonded on this element body. Furthermore, atleast one surface of each of this pair of the electrode foils isroughened, and in addition, this roughened surface is applied with theAu (gold) flash plating, and the pair of the electrode foils are bondedon both surfaces of the element body with the surfaces thereof, whichare applied with the Au flash platings, being faced with the elementbody.

[0012] In short, the Au flash plating is applied on the roughenedsurface of each of the electrode foils to provide a layer made of gold,which is a stable material with low electrical resistance, in theconnecting portion between the element body and the electrode foil ofthe polymer PTC element according to this aspect. This not only lowersthe electrical resistance in this connecting portion sufficientlycompared with a conventional example, but also reduces changes due toits environment and aging to eliminate increase in the electricalresistance in the connecting portion.

[0013] As a result, a stable electrical characteristic is obtainable,and furthermore, a low-cost electrolytic copper foil can be used,thereby enabling reduction in production cost of the polymer PTCelement.

[0014] According to another aspect of the present invention, provided isa polymer PTC element comprising: an element body including a polymerand a conductive material dispersively mixed in this polymer; and a pairof electrode foils which are bonded on both surfaces of this elementbody respectively and in which surfaces thereof opposite this elementbody are applied with Au flash platings respectively.

[0015] The polymer PTC element as described above brings about thefollowing effect.

[0016] The polymer PTC element according to this aspect is so structuredthat it comprises the element body including the polymer, and theconductive material dispersively mixed in this polymer and the pair ofthe electrode foils are bonded on this element body. Furthermore, the Auflash plating is applied on the surface of each of the electrode foilsnot facing this element body, namely, the surface of each of theelectrode foils opposite the element body.

[0017] More specifically, as surface treatment of an electrode foil of aconventional polymer PTC element, soft-solder plating is employed,taking solderability between this electrode foil and an externalconnecting terminal such as a lead terminal into consideration. Then,after the electrode foil is bonded on an element body, soft-solderleveling is performed. However, thermal damage to the element body isworried about in this soft-solder leveling since the electrode foilcomes in contact with a soft-solder melting portion whose temperature is200° C. to 220° C.

[0018] Meanwhile, in soft-soldering the electrode foil with the externalconnecting terminal, it can be considered to employ electricalsoft-solder plating including an Sn plating. However, in this case,though this electrical soft-solder plating can be performed more easilyand costs lower when they are tumble-plated in the individual productform of the element body, the plating thickness needs to be 1 μm or morein this tumble plating, considering solderability.

[0019] When the plating is carried out under the condition of theplating thickness of 1 μm or more, the plating also adheres to a portionof the element body exposed in an end surface since the element bodyalso has some degree of electrical conductivity, which may possiblyresult in production of a poor product.

[0020] On the other hand, in the Au flash plating according to thisaspect, the temperature does not become high during the treatment, andtherefore, there is no possibility of causing a poor characteristic ofthe element body due to thermal degradation. Moreover, solderability canbe satisfied with a relatively small plating thickness of about 0.01 μmto about 0.1 μm, which prevents the plating from adhering to the endsurface of the element body, thereby preventing the element body frombeing a poor product. Moreover, the characteristic of the polymer PTCelement is not degraded due to a plating solution or the treatmenttemperature while the plating is applied. In addition, the small platingthickness can realize improvement in a process yield and relatively lowcost production.

[0021] According to still another aspect of the present invention,provided is a polymer PTC element comprising: an element body includinga polymer and a conductive material dispersively mixed in this polymer;and a pair of electrode foils which are bonded on both surfaces of theelement body respectively and in which at least a surface of each of theelectrode foils facing the element body is roughened and both surfacesthereof are applied with Au flash platings respectively.

[0022] The polymer PTC element as described above brings about thefollowing effect.

[0023] The polymer PTC element according to this aspect is so structuredthat it comprises the element body including the polymer and theconductive material dispersively mixed in this polymer and the pair ofthe electrode foils are bonded on this element body. Furthermore, atleast the surface of each of the pair of the electrode foils facing theelement body is roughened and both surfaces of the electrode foils areapplied with the Au flash platings.

[0024] More specifically, in a case the electrode foil is intended to beplated as a single object before the electrode foil is bonded on theelement body, if soft-solder plating is employed, a soft solder adheresto the roughened surface, so that a poor characteristic such as decreasein bonding strength may possibly be caused. Therefore, one surface ofthe electrode foil must be completely masked during the treatment.

[0025] On the other hand, the Au flash plating according to this aspectnot only eliminates the possibility of causing a poor characteristicowing to a thin plating thickness but also makes it possible tosynchronously plate both of the surfaces, namely, the roughened surfaceto be bonded on the element body and the surface on which the externalconnecting terminal is connected. Especially, when considering theaspect of the present invention described above, gold can adhere to bothof the surfaces of the electrode foil synchronously by one platingprocess, thereby simplifying production processes and further reducingproduction cost of the polymer PTC element.

BRIEF DESCRIPTION OF THE DRAWINGS

[0026]FIG. 1 is a perspective view showing a polymer PTC elementaccording to one embodiment of the present invention;

[0027]FIG. 2 is a graph showing the correlation between resistance andtemperature of an element body in a first example of the polymer PTCelement according to the embodiment of the present invention;

[0028]FIG. 3 is a graph showing the correlation between resistance andtemperature of an element body in a second example of the polymer PTCelement according to the embodiment of the present invention;

[0029]FIG. 4A is a cross sectional view showing a manufacture of thepolymer PTC element according to the embodiment of the present inventionand showing a state before the element body and electrode foils areconnected with each other, and FIG. 4B is a cross sectional view showingthe manufacture of the polymer PTC element according to the embodimentof the present invention and showing a state after the element body andthe electrode foils are connected with each other;

[0030]FIG. 5A is a perspective view showing the manufacture of thepolymer PTC element according to the embodiment of the present inventionand showing the state after the element body and the electrode foils areconnected with each other, FIG. 5B is a perspective view showing themanufacture of the polymer PTC element according to the embodiment ofthe present invention and showing a state before lead terminals areconnected onto the electrode foils, and FIG. 5C is a perspective viewshowing the manufacture of the polymer PTC element according to theembodiment of the present invention and showing a state after the leadterminals are connected onto the electrode foils;

[0031]FIG. 6 is a graph showing the increase in electrical resistivityafter thermo cycle tests are conducted on samples of the polymer PTCelement; and

[0032]FIG. 7 is a perspective view showing a polymer PTC elementaccording to a conventional art.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0033] An embodiment of a polymer PTC element according to the presentinvention is explained below with reference to the drawings to clarifythe present invention.

[0034]FIG. 1 is a perspective view showing a polymer PTC element 10according to this embodiment. In this embodiment, a plate-like elementbody 12 constituted of a polymer and a conductive material dispersivelymixed in this polymer constitutes a body portion of the polymer PTCelement 10 shown in this drawing.

[0035] Note that an element body inclusive of a polymer synthesizedusing a metallocene catalyst and of conductive granules dispersivelymixed in this polymer as a conductive material and having spike-shapedprotrusions is employed as the element body 12 in the polymer PTCelement 10 according to this embodiment, so that temperature change in arelatively low range of temperature can be detected. Specifically, thetemperature of a resistance changing point of this element body 12 isset within a relatively low range of 60° to 120° C.

[0036] Moreover, electrode foils 14 for passing electricity areprovided, being bonded on both of upper and lower surfaces of thiselement body 12 respectively. Surfaces of a pair of these electrodesfoils 14 facing the element body 12, namely, surfaces bonded on theelement body 12, are roughed to be surfaces 14A, and in addition, Au(gold) flash platings are applied on these surfaces facing the elementbody 12. The Au flash platings are also applied on surfaces 14B of thepair of these electrode foils 14 not facing the element body 12respectively in order to enhance solderability.

[0037] As described above, at least the respective surfaces of the pairof the electrode foils 14 facing the element body 12 are roughed to bethe surfaces 14A, and the Au flash platings are applied on both of thesurfaces of the pair of these electrode foils 14 respectively.Incidentally, the pair of these electrode foils 14 are formed ofelectrolytic copper foils or electrolytic nickel foils and have athickness of about 20 μm to about 40 μm, with the roughened surfaceshaving a surface roughness Ra of about 1.0 μm to about 1.5 μm, and theAu platings having a thickness of about 0.01 μm to 0.1μm.

[0038] Meanwhile, the pair of these electrode foils 14 bonded on theelement body 12 respectively and a pair of lead terminals 16, eachformed in a long and narrow plate shape, are connected by Sn (tin)—Bi(bismuth) alloy soft solders 18 respectively. In other words, the pairof the lead terminals 16 are connected on the upper and lower surfacesof the element body 12 respectively via the electrode foils 14 toconstitute the polymer PTC element 10.

[0039] Next, it is explained why the element body inclusive of thepolymer synthesized using the metallocene catalyst and of the conductivegranules dispersively mixed in this polymer as the conductive materialand having the spike-shaped protrusions is employed as the element body12 according to this embodiment.

[0040] As characteristics required for the element body 12 of thepolymer PTC element 10, such characteristics can be listed that roomtemperature resistivity is sufficiently low in its non-operating stateat room temperature, that a change rate of resistivity in its operatingstate relative to the room temperature resistivity is sufficiently high,and that repeated operations cause a little change in the resistivity.

[0041] As a thermoplastic crystalline macromolecule which is a polymerfor the element body 12 of the polymer PTC element 10, high-crystal andhigh-density polyethylene has been mainly used conventionally. This isbecause a higher-crystal macromolecule has a higher expansivity and ahigher resistance change rate can be obtained in it. On the other hand,a lower-crystal macromolecule has a lower crystallization speed, whichdisables it to return to the original crystal state when it is cooledafter melted, and the change in resistivity at room temperature is big,and therefore, it is difficult to employ it for the element body 12 dueto its property.

[0042] However, one of the disadvantages of using the high-densitypolyethylene is its high operating temperature. Specifically, theoperating temperature of the polymer PTC element when used as anovercurrent protective element becomes about 130° C. which is itsmelting point, and thermal influence to other electronic components on acircuit substrate cannot be sometimes ignored at this temperature.Furthermore, the operating temperature is also too high as an overheatprotective component of a secondary battery.

[0043] Therefore, it has been decided to especially use linearlow-density polyethylene (LLDPE) which is a polymer polymerized usingthe metallocene catalyst, in order to make it possible to maintain agood resistance recovery characteristic while maintaining the operatingtemperature at about 100° C. which is lower than that of thehigh-density polyethylene whose operating temperature is high.

[0044] This is because a small amount of low-density, low-molecularingredient due to a narrow width of molecule distribution of the polymerwhich is brought about by the polymerization using this metallocenecatalyst is considered to be one of the reasons the resistance recoverycharacteristic can be maintained. Specifically, a high-densityingredient is crystallized in a conventionally and generally used linearlow-density polyethylene and crystallization is promoted with thisingredient as a crystal nucleus. On the other hand, in the polymersynthesized using the metallocene catalyst, it can be thought that, evenwhen the polymer PTC element operates to melt the crystal, acharacteristic change after that becomes small to reduce resisitivitychange at room temperature since a crystal nucleus is uniformly made andgrown.

[0045] As described above, the polymer synthesized using the metallocenecatalyst is employed so that the operating temperature can be made lowerthan that of a conventional polymer PTC element. Accordingly, an elementhaving a stable characteristic while maintaining low operatingtemperature, which has been conventionally difficult to realize, isobtainable.

[0046] Furthermore, the use of the conductive granules having thespike-shaped protrusions makes it possible to obtain a low roomtemperature resistance and a high resistance change rate at the sametime.

[0047] Specifically, since the conductive granules having thespike-shaped protrusions are used, its shape allows a tunnel current toeasily pass therethrough, so that the element body 12 with a relativelylow room temperature resistance can be obtained compared with an elementusing spherical conductive granules. In addition, intervals between theconductive granules are large compared with those of the sphericalconductive granules so that a larger resistance change can be obtainedin its operating state.

[0048] A graph showing the correlation between resistance andtemperature of materials which can be used as the element body 12according to this embodiment in a first example is shown in FIG. 2, anda graph showing the correlation between resistance and temperature ofthe same in a second example is shown in FIG. 3.

[0049] The first example shown in FIG. 2 out of these drawings presentshysteresis between heating and cooling, but it is in a tolerable rangefor this material to be used as the element body 12. Meanwhile, a lowmolecular organic compound is mixed in this material and this lowmolecular organic compound is made to be an operating material, so thata transition temperature (operating temperature) at which resistanceincreases at the time of heating can be made substantially equal to thetemperature at which the resistance returns to a low value at the timeof cooling, and a material having the characteristic shown in the graphin FIG. 3 can be obtained.

[0050] Next, production processes of the polymer PTC element 10according to this embodiment are explained.

[0051] First, the element body 12 constituted of the polymer and theconductive material dispersively mixed in this polymer is made.

[0052] Meanwhile, one surface of each of the pair of the electrode foils14 shown in FIG. 4A is roughened with the surface roughness Ra of about1.0 μm to about 1.5 μm to be the surface 14A, and at the same time, theAu flash platings are applied on both surfaces of each of the foils 14respectively with plating thickness of about 0.01 μm to about 0.1 μm.Then, while the element body 12 is sandwiched between the pair of theelectrode foils 14, they are pressed, for example, by a not-shown pressmachine. Thereby, the pair of the electrode foils 14 can be bonded onthe element body 12 in a manner that the surface 14A roughened andapplied with the Au flash plating, out of both surfaces of each of thepair of the electrode foils 14, faces the element body 12 as shown inFIG. 4B and FIG. 5A.

[0053] As a result, the pair of the electrode foils 14 are in a state ofbeing disposed on the upper and lower surfaces of the element body 12respectively, and in this state, pasted Sn—Bi alloy soft solders 18 areapplied on the electrode foil 14 sides as shown in FIG. 5B (theapplication only on one of the electrode foils 14 is shown). Through theabove-described processes, the aforesaid Au flash platings are alsoapplied on surfaces 14B of the electrode foils 14 which are opposite theelement body 12 and on which the Sn—Bi alloy soft solders 18 areapplied. However, the pasted Sn—Bi alloy soft solders 18 may be appliedon the lead terminal 16 sides.

[0054] In the above-described processes, electrolytic copper foils orelectrolytic nickel foils, each having one surface roughened, may beused as the electrode foils 14, and foils subjected to roughening andelectrolytic nickelic treatment may be used as the electrolytic copperfoils.

[0055] Thereafter, the pasted Sn—Bi alloy soft solders 18 are oncemelted by reflowing to soft-solder the electrode foils 14 and the leadterminals 16 by the Sn—Bi alloy soft solders 18 and connect them asshown in FIG. 5C, so that the polymer PTC element 10 is finished.

[0056] Specifically, the temperature of the resistance changing point ofthe element body 12, which is a body portion of this polymer PTC element10, is in a relatively low temperature range of 60° C. to 120° C., sothat the element body 12 is susceptible to thermal influence at the timeof this reflowing. However, the melting point of the Sn—Bi alloy softsolders 18 is also in a low temperature range of 139° C. to 160° C., sothat the thermal influence given to the element body 12 due to the softsoldering between the electrode foils 14 and-the lead terminals 16 issmall.

[0057] Incidentally, as a concrete example of the Sn—Bi alloy softsolder 18 in this embodiment, it can be considered to use a lead-freesoft solder including Sn, Bi, and Ag (silver), with Bi content being 47%to 64%, Ag content being 0% to 3%, and Sn content being the rest. Thereflow peak temperature at the time of the reflowing of this Sn—Bi alloysoft solder 18 is made to be about 160° C. to about 180° C.

[0058] Next, the operation of the polymer PTC element 10 according tothis embodiment is explained.

[0059] In this embodiment, the element body 12 includes the polymer andthe conductive material dispersively mixed in this polymer, in whichresistance change occurs in the low temperature range of 60° C. to 120°C. The element body 12 is so structured that the pair of the electrodefoils 14 are bonded on both of the surfaces of this element body 12respectively and the pair of these electrode foils 14 and the pair ofthe lead terminals 16 are connected by the Sn—Bi alloy soft solders 18respectively.

[0060] Moreover, at least one surface of each of the pair of theseelectrode foils 14 is roughened, and in addition, the Au flash platingsare applied on the surfaces 14A which are the roughened surfaces. Then,the electrode foils 14 are bonded on the element body 12 in a mannerthat the roughened surfaces 14A applied with the Au flash platings facethe element body 12. Note that the Au flash platings are also applied onthe surfaces 14B of the electrode foils 14 which do not face thiselement body 12, namely, the surfaces opposite this element body 12 inthis embodiment, in order to satisfy solderability.

[0061] In other words, the Au flash platings are applied on theroughened surfaces 14A of the electrode foils 14 so that layers of goldwhich is a stable material with low electrical resistance are disposedin the connecting portions between the element body 12 and the electrodefoils 14 of the polymer PTC element 10 according to this embodiment.This not only sufficiently lowers the electrical resistance in theseconnecting portions compared with the conventional example but alsolessens changes due to its environment and aging so that increase in theelectrical resistance in the connecting portions is prevented.

[0062] As a result, a stable electrical characteristic is obtainableand, in addition, a low-cost electrolytic copper foil can be used sothat the production cost of the polymer PTC element 10 can be reduced.

[0063] Meanwhile, according to this embodiment, the Au flash platingswith a relatively thin plating thickness of about 0.01 μm to about 0.1μm are also applied on the surfaces 14B of the electrode foils 14opposite the element body 12, as described above. Therefore, thetemperature of the Au flash platings do not become high at the time ofthe plating so that there is no possibility of causing a poorcharacteristic of the element body 12 due to thermal degradation.Furthermore, since the plating thickness may be small as describedabove, the plating does not adhere to the end surface of the elementbody 12, thereby preventing production of a poor product.

[0064] Moreover, the characteristic of the polymer PTC element 10 is notdegraded either due to a plating solution or the treatment temperatureat the time of the plating, and in addition, since the plating thicknessis small, improvement in a process yield and relatively low-costproduction can be realized.

[0065] As described above, in this embodiment, at least the surfaces ofthe electrode foils 14 facing the element body 12 are roughened to bethe surfaces 14A and the Au flash platings are applied on both of thesurfaces of the electrode foils 14, namely, the surfaces 14A and 14B.Consequently, both of the surfaces, namely, the roughened surfaces 14Abonded on the element body 12 and the surfaces 14B onto which the leadterminals 16 are connected are plated respectively.

[0066] More specifically, a soft-solder plating may possibly cause apoor characteristic since thick soft solders adhere to the roughenedsurfaces. Therefore, it is necessary to completely mask one surface ofeach of the electrode foils during the treatment, while the Au flashplating employed in this embodiment does not have a possibility ofcausing a poor characteristic owing to its thin plating.

[0067] As a result, it is possible to have gold adhere to both of thesurfaces of each of the electrode foils 14 at the same time by oneplating process, so that the production processes are simplified and theproduction cost of the polymer PTC element 10 can be further reduced.

[0068] Meanwhile, in this embodiment, the element body 12 includes thepolymer and the resistance change thereof occurs within the lowtemperature range of 60° C. to 120° C. Consequently, the element body 12is easily affected by heat. But the electrode foils 14 disposed on thiselement body 12 and the lead terminals 16 are connected with each otherby the Sn—Bi alloy soft solders 18 with a low melting point of 139° C.to 160° C. Therefore, as a result of employing the Sn—Bi alloy softsolders 18, the electrode foils 14 and the lead terminals 16 can beconnected by the reflowing, which is common as a soft soldering process,and in addition, they can be connected while the reflow peak temperatureof the soft solder is maintained within a low temperature range of 160°C. to 180° C.

[0069] As a result, the thermal influence given to the element body 12is reduced, thereby eliminating the possibility of causing a poorcharacteristic due to thermal degradation. In other words, the elementbody 12 according to this embodiment is easily affected by heat, and thedegradation of its characteristic is prominent especially when heat atthe temperature of 190° C. or higher is given to it, but since thereflow peak temperature of the soft solder is also in the lowtemperature range of 160° C. to 180° C., no characteristic degradationdue to heat is caused.

[0070] Furthermore, the maximum value of the usable temperature range ofthe polymer PTC element 10 is also suppressed to be about 120° C. lowerthan 139° C., which is the melting point of the soft solder of 57% Biwhose melting point is the lowest. Therefore, no problem involved inconnection such as melting of the soft solder when the polymer PTCelement 10 is used does not occur.

[0071] Incidentally, the melting point of an ordinary eutectic softsolder which is Sn-37Pb is 183° C. and the reflow peak temperature whenit is treated, is about 200° C. to about 220° C. Therefore, thetreatment temperature is greatly higher compared with a case when theSn—Bi alloy soft solder 18 is used as in this embodiment, which gives agreat damage to the element body 12 and therefore, this eutectic softsolder is not usable.

[0072] As described above, in the polymer PTC element 10 according tothis embodiment, the soft soldering process can also be carried out bythe ordinary reflowing when the electrode foils 14 and the leadterminals 16 are soft-soldered. Furthermore, influence and deformationby heat at the time of the soft soldering is small, so that not onlyreliability is further enhanced but also a process yield is enhanced. Asa result, the polymer PTC element 10 becomes low-priced.

[0073] Next, test results concerning stability of electrical resistanceof the polymer PTC element according to the present invention arespecifically explained.

[0074] First, samples of five kinds of polymer PTC elements are made fortesting, a plurality of samples being prepared for each kind. Samples 1are examples in each of which an electrolytic nickel foil with onesurface thereof being roughened is used as an electrode foil. Samples 2are examples in each of which an electrolytic copper foil is subjectedto surface-roughening and electrolytic nickelic treatment as anelectrode foil. Samples 3 are examples in each of which an electrolyticcopper foil with one surface thereof being roughened is applied with anAu flash plating as an electrode foil. Samples 4 are examples in each ofwhich an electrolytic nickel foil with one surface thereof beingroughened is applied with an Au flash plating as an electrode foil.Samples 5 are examples in each of which a foil made by subjecting anelectrolytic copper foil to surface-roughening and electrolytic nickelictreatment is applied with an Au flash plating as an electrode foil.Therefore, the samples 3 to the samples 5 among the above samples aresamples corresponding to the polymer PTC element of the presentinvention.

[0075] Increase in electrical resistivity after a 100-hour thermo cycletest (a thermo cycle condition: −40° C. to +85° C.) is conducted on eachof the above-described samples is shown in the graph in FIG. 6. Morespecifically, the graph in this drawing shows the increase range of theelectrical resistivity of each of the samples after the thermo cycletest, assuming that an initial value of the samples 1 (specifically, 10mΩ) is 100.

[0076] The result of the test shows that the increase ranges areconcentrated in the range of 200 to 350 in the samples 1 and 2, whilethe increase ranges are concentrated in the range of 150 to 250 in thesamples 3 to 5. Therefore, the change rate of the electrical resistivityof the samples 3 to 5 corresponding to the polymer PTC element accordingto the present invention is about half the change rate of the electricalresistivity of the samples 1 and 2, and therefore, it is confirmed thatthe samples 3 to 5 are very stable.

[0077] Next, the test result concerning thickness and strength of the Auflash plating of the polymer PTC element according to the presentinvention is specifically explained.

[0078] First, the correlation between the thickness of the Au flashplating on the surface of the electrode foil which is soft-soldered andthe bonding strength of the soft soldering is tested.

[0079] As a result of this test, in samples in which the Au flashplating is 0.02 μm to 0.05 μm in thickness (average 0.03 μm), the leadterminals themselves are cut away at about 150 newtons when they arepulled by forces in the directions of the arrows F shown in FIG. 1.

[0080] On the other hand, in samples in which the Au flash plating is0.2 μm to 0.5 μm in thickness (average 0.3 μm), abrasion occurs insoft-solder bonding portions at about 100 newtons when they aresimilarly pulled by the force in the directions of the arrows F shown inFIG. 1.

[0081] Generally, when the Au flash plating is applied, the bondingstrength may possibly be lowered due to the dispersion of Au into thesoft solder, but the result shows that the plating thickness of about0.02 μm to about 0.05 μm as described above suppresses the dispersionamount to be small and the influence to the bonding strength of the softsoldering to be small.

[0082] The correlation between the thickness of the Au flash plating onthe surface of the electrode foil facing the element body and thebonding strength between the element body and the electrode foil istested.

[0083] As a result of this test, in the samples in which the Au flashplating is 0.02 μm to 0.05 μm in thickness (average 0.03 μm), abrasionoccurs at about 10 newtons when forces in the directions of the arrows Fshown in FIG. 4B are given thereto. At this time, the element bodiesthemselves are broken, and parts of the element bodies are left bondedon many of the surfaces of the electrode foils facing the elementbodies.

[0084] On the other hand, in the samples in which the Au flash platingis 0.2μm to 0.5 μm in thickness (average 0.3 μm), abrasion occurs atabout eight newtons when the forces in the directions of the arrows Fshown in FIG. 4B are similarly given thereto. At this time, some of theelement bodies are broken, but the part of the element body left bondedon the surface of the electrode foil facing the element body is smallerthan that in the above-described case.

[0085] From the above results, it can be considered that properthickness of the Au flash plating is 0.1 μm or less, and morepreferably, the plating thickness is about 0.02 μm to about 0.05 μm,since the bonding strength is lowered when the plating thickness is 0.2μm or more.

[0086] Incidentally, in this embodiment, the surfaces of the electrodefoils facing the element body are roughened, and in roughening thesurfaces of the electrode foils, it can be considered that protrudingportions among recessed and protruding portions constituting theroughened surfaces are formed, for example, in a hook shape. The heightof 1 μm or more is sufficient for the hook-shaped protruding portions,and it can be considered that it is made to be, for example, about 4 μmto about 10 μm.

[0087] Furthermore, the protruding portions out of the protruding andrecessed portions constituting the roughened surfaces can be formed in ahook shape by having granular materials adhere to the surfaces of theelectrode foils. Incidentally, it can be considered as treatment forroughening the surfaces of the electrode foils by forming the protrudingportions in a hook shape that the surfaces are roughened by treatmentsuch as etching and plating, and thereafter, they are electrolyzed sothat an ionized material in an electrolytic solution adheres to topportions of the protruding portions of the roughened surfaces. Thisfacilitates the forming of the protruding portions to be hook-shapedprotruding portions.

[0088] Meanwhile, the lead-free soft solder including Sn, Bi, and Ag isused as the Sn—Bi alloy soft solder 18, Bi content being 47% to 64%, Agcontent being 0% to 3%, and Sn content being the rest. Therefore, sincelead is not included therein, the influence given to the environment issmall. Moreover, when it includes 3% Ag or less, for example, about 1%,and about 57% Bi, mechanical reliability such as the connecting strengthby the soft solder is enhanced. Therefore, it is preferable that how thesoft solder is constituted is determined in more detail based onproduction cost, the structure of the polymer PTC element 10,reliability data, and so on.

[0089] Incidentally, in the above-described embodiment, Bi content,namely, bismuth content is in a range of 47% to 64%, Ag content, namely,silver content is 3% or less, and Sn content, namely, tin content is therest. But an alloy soft solder only of tin-bismuth, non-inclusive ofsilver may also be used. % signifying their content is WT %.

[0090] According to the polymer PTC element of the present invention,the electrical resistance in the connecting portion between the elementbody and each of the electrodes is sufficiently low, and in addition,the electrical resistance in this connecting portion is not increaseddue to environmental and aging changes and so on, so that it is given astable electrical characteristic. Furthermore, not only the possibilityof causing a poor characteristic due to thermal degradation iseliminated, but also solderablity of the lead terminal is satisfied.

What is claimed is:
 1. A polymer PTC element, comprising: an elementbody including a polymer and a conductive material dispersively mixed inthis polymer; and a pair of electrode foils, in each of which at leastone surface is roughened and this surface is applied with an Au flashplating and which are bonded, with these surfaces being faced with theelement body, on both surfaces of the element body respectively.
 2. Thepolymer PTC element according to claim 1, wherein the Au flash platingapplied on each of the electrode foils has a thickness of 0.01 μm to 0.1μm.
 3. A polymer PTC element, comprising: an element body including apolymer and a conductive material dispersively mixed in this polymer;and a pair of electrode foils which are bonded on both surfaces of theelement body respectively and in which surfaces opposite the elementbody are applied with Au flash platings respectively.
 4. The polymer PTCelement according to claim 3, wherein the Au flash plating applied oneach of the electrode foils has a thickness of 0.01 μm to 0.1 μm.
 5. Thepolymer PTC element according to claim 3, wherein the Au flash platingapplied on each of the electrode foils has a thickness of 0.02 μm to0.05 μm.
 6. The polymer PTC element according to claim 3, wherein a leadterminal is attached on the surface of the electrode foil opposite theelement body by an alloy soft solder.
 7. The polymer PTC elementaccording to claim 3, wherein a pair of lead terminals are attached onthe surfaces of the pair of the electrode foils opposite the elementbody respectively by an alloy soft solder.
 8. A polymer PTC element,comprising: an element body including a polymer and a conductivematerial dispersively mixed in this polymer; and a pair of electrodefoils which are bonded on both surfaces of the element body respectivelyand in each of which at least a surface facing the element body isroughened and both surfaces are applied with Au flash platingsrespectively.
 9. The polymer PTC element according to claim 8, whereinthe Au flash plating applied on each of the electrode foils has athickness of 0.01 μm to 0.1 μm.
 10. The polymer PTC element according toclaim 8, wherein the Au flash plating applied on each of the electrodefoils has a thickness of 0.02 μm to 0.05 μm.
 11. The polymer PTC elementaccording to claim 8, wherein a lead terminal is attached on the surfaceof the electrode foil opposite the element body by an alloy soft solder.